Heat leaves the body in a whirlwind of vortices, and a good jacket provides warmth by keeping those vortices from drifting off. The industry standard, 600-fill goose down, works because the feathers cling together, forming a layer of insulation that blocks the vortices and keeps the heat generated by the body inside the coat. But down can be bulky and doesn't perform well when wet, which is why climbers and extreme athletes have long sought lighter outerwear that's impervious to precipitation. Enter The North Face's new ThermoBall coat, which is filled with .15-inch-diameter polyester balls that cluster together, similar to the way down feathers do. Unlike down, though, ThermoBalls don't absorb water. The ThermoBall coat is the company's lightest (11 ounces), compresses into its own pocket and provides gooselike warmth. Those qualities make it worth a gander.